Palladium particle-catalyzed selective butadiene hydrogenation: effect of covalent organic framework modification

Surface modifications of active metal species can usually alter their electronic properties and impact their performance in catalytic applications. Herein, alumina-supported palladium nanoparticles with and without covalent organic framework (COF) modification were prepared and their catalytic performance in the selective hydrogenation of 1,3-butadiene were studied. The COF-modified catalyst showed lower activity than the pristine analogue; however, it could afford higher selectivity to butenes. Thorough characterizations, including X-ray photoelectron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), revealed that COF deposition preferentially occurred on the extended Pd surfaces instead of the terrace sites and did not significantly change the Pd particle sizes, thus leading to more charged Pd delta+ species. Kinetic study further suggested a much enhanced adsorption of butadiene compared to H2. Competitive adsorption between the two reactants may account for the reduced activity of the modified catalyst, as substantiated by the higher apparent energy barrier for the modified catalyst. In contrast, the elevated selectivity for butenes could be attributed to their rapid desorption from the active Pd centers. Deposition of a covalent organic framework on palladium nanoparticles induces geometric and electronic effects on the metal species and improves their selective performance in 1,3-butadiene hydrogenation.